Wireless sound transmission system and method

ABSTRACT

A system for providing sound to at least one user, having at least one audio signal source for providing audio signals; a transmission unit with a digital transmitter for wirelessly transmitting the audio signals as data packets; at least one receiver unit with at least one digital receiver for reception of the audio signals from the transmission unit; a mechanism for stimulating the hearing of the user(s) according to audio signals supplied from the receiver unit. The transmission unit transmits each data packet in a separate slot of a TDMA frame at a different frequency in a frequency hopping sequence, in at least some of the slots, the audio signals are transmitted as audio data packets, the same audio packet being transmitted at least twice in the same TDMA frame without expecting acknowledgement, and the TDMA frames being structured for unidirectional broadcast transmission of the audio data packets.

The present application is a reissue application of U.S. Pat. No.9,137,613 filed Feb. 12, 2010, which is a National Stage ofInternational Application No. PCT/EP2010/051812 filed Aug. 9, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a system and a method for providing sound to atleast one user, wherein audio signals from an audio signal source, suchas a microphone for capturing a speaker's voice, are transmitted via awireless link to a receiver unit, such as an audio receiver for ahearing aid, from where the audio signals are supplied to means forstimulating the hearing of the user, such as a hearing aid loudspeaker.

2. Description of Related Art

Presently, in such systems, the wireless audio link usually is an FM(frequency modulation) radio link. According to a typical application ofsuch wireless audio systems, the receiver unit is connected to orintegrated into a hearing instrument, such as a hearing aid, with thetransmitted audio signals being mixed with audio signals captured by themicrophone of the hearing instrument prior to being reproduced by theoutput transducer of the hearing instrument. The benefit of such systemsis that the microphone of the hearing instrument can be supplemented orreplaced by a remote microphone which produces audio signals which aretransmitted wirelessly to the FM receiver, and thus, to the hearinginstrument. In particular, FM systems have been standard equipment forchildren with hearing loss in educational settings for many years. Theirmerit lies in the fact that a microphone placed a few centimeters fromthe mouth of a person speaking receives speech at a much higher levelthan one placed several feet away. This increase in speech levelcorresponds to an increase in signal-to-noise ratio (SNR) due to thedirect wireless connection to the listener's amplification system. Theresulting improvements of signal level and SNR in the listener's ear arerecognized as the primary benefits of FM radio systems, ashearing-impaired individuals are at a significant disadvantage whenprocessing signals with a poor acoustical SNR.

A typical application of such wireless audio systems is at school,wherein the teacher uses a wireless microphone for transmitting thecaptured audio signals via the transmission unit to receiver units wornby the students. Since the receiver units and the respective hearingaids are usually owned by the students, the receiver units may be ofdifferent types within a class.

Another typical application of wireless audio systems is the case inwhich the transmission unit is designed as an assistive listeningdevice. In this case, the transmission unit may include a wirelessmicrophone for capturing ambient sound, in particular from a speakerclose to the user, and/or a gateway to an external audio device, such asa mobile phone; here the transmission unit usually only serves to supplywireless audio signals to the receiver unit(s) worn by the user.

Examples of analog wireless FM systems particularly suited for schoolapplications are described, for example, in European Patent ApplicationEP 1 863 320 A1 and International Patent Application Publication WO2008/138365 A1. According to these systems, the wireless link not onlyserves to transmit audio signals captured by the wireless microphone,but in addition, also serves to transmit control data obtained fromanalyzing the audio signals in the transmission unit to the receiverunit(s), with such control data being used in the receiver unit toadjust, for example, the gain applied to the received audio signalsaccording to the prevailing ambient noise and the issue of whether thespeaker is presently speaking or not.

In applications where the receiver unit is part of or connected to ahearing aid, transmission is usually carried out by using analog FMtechnology in the 200 MHz frequency band. In recent systems, the analogFM transmission technology is replaced using digital modulationtechniques for audio signal transmission. An example of such digitalsystem is available from the company Comfort Audio AB, 30105 Halmstad,Sweden under the COMFORT DIGISYSTEM® trademark.

A specific example of an analog wireless FM system particularly suitedfor school applications is described in International Patent ApplicationPublication WO 2008/074350 A1, wherein the system consists of aplurality of transmission units comprising a microphone and a pluralityof analog FM receiver units and wherein only one of the transmissionunits has an analog audio signal transmitter, while each of thetransmission units is provided with a digital transceiver in order torealize an assistive digital link for enabling communication between thetransmission units. The assistive digital link also serves to transmitaudio signals captured by a transmission unit not having the analogtransmitter to the transmission unit having the analog transmitter fromwhere the audio signals are transmitted via the analog FM link to thereceiver units.

U.S. Patent Application Publication 2002/0183087 A1 relates to aBluetooth link for a mobile phone using two parallelantennas/transceivers, wherein each data packet is sent once and whereinfor a sequence of packets, usually for the next 8 packets, a certain oneof the antennas is selected according to previous channel qualitymeasurements as a function of frequency. For each packet of the sequenceone of the antennas is selected depending on the respective frequency atwhich the packet is to be transmitted, wherein the frequency isdetermined by a frequency hopping sequence.

U.S. Patent Application Publication 2006/0148433 A1 relates to awireless link between a mobile phone and a base station of the mobilenetwork, wherein two receivers are used in parallel for achievingdiversity if the coverage is poor.

Canadian Patent 2 286 522 C relates to a diversity radio receptionmethod, wherein two data packets received in parallel by two receiversare compared and, if they differ from each other, the more reliable oneis selected for further processing.

In the publication “Effect of Antenna Placement and Diversity onVehicular Network Communications” by S. Kaul, K. Ramachandran, P.Shankar, S. Oh, M. Gruteser, I. Seskar, T. Nadeem, 4^(th) Annual IEEECommunications Society Conference on Sensor, Mesh and Ad HocCommunications and Networks, 2007, SECON '07, pp. 112-121, a packetlevel diversity approach is described, wherein in a vehicle-to-vehiclelink using roof- and in-vehicle-mounted omni-directional antennas andIEEE 802.11a radios operating in the 5 GHz band a packet level selectiondiversity scheme using multiple antennas and radios is utilized toimprove performance not only in a fading channel but also inline-of-sight conditions. A similar approach is used in “Packet-LevelDiversity—From Theory to Practice: An 802.11-based ExperimentalInvestigation” by E. Vergetis et al., MobiCom '06 (see alsohttp://repository.upenn.edu/ese_papers/194), wherein a packet leveldiversity scheme is applied to a wireless data link between a laptopcomputer and an access point.

A presentation by S. Shellhammer “SCORT—An Alternative to the BluetoothSCO Link for Voice Operation in an Interference Environment” documentIEEE 802.15-01/145r1, March 2001, and the IEEE P802.15 Working Group forWireless Personal Area Networks, relates to a proposed alternative forthe Bluetooth SCO link for operation in an interference environment,wherein it is proposed to use, in a bi-directional point-to-point link(i.e. full duplex link) for voice transmission, repeated transmission ofthe same audio packet without involving a receipt acknowledgement by thereceiving device.

U.S. Patent Application Publication 2007/0009124 A1 and correspondingU.S. Pat. No. 7,778,432 B2 relate to a wireless network forcommunication of binaural hearing aids with other devices, such as amobile phone, using slow frequency hopping, wherein each data packet istransmitted in a separate slot of a TDMA frame, with each slot beingassociated to a different transmission frequency, wherein the hoppingsequence is calculated using the ID of the master device, the slotnumber and the frame number. A link management package is sent from themaster device to the slave devices in the first slot of each frame. Thesystem may be operated in a broadcast mode. Each receiver is turned ononly during the transmission during time slots associated to therespective receiver. The system has two acquisition modes forsynchronization, with two different handshake protocols. Eight LMPmessages are transmitted in every frame during initial acquisition, andone LMP message is transmitted in every frame once a network isestablished. Handshake, i.e. bi-directional message exchange, is neededboth for initial acquisition and acquisition into the establishednetwork. During acquisition, only a reduced number of acquisitionchannels is used, with the frequency hopping scheme being applied tothese acquisition channels. The system operates in the 2.4 GHz ISM band.A similar system is known from U.S. Patent Application Publication2009/0245551 A1 and corresponding U.S. Pat. No. 8,229,146 B2.

U.S. Pat. No. 7,532,610 B2 relates to an adaptive frequency hoppingscheme, wherein bad frequencies are empirically excluded from thefrequency range used by the frequency hopping algorithm.

International Patent Application Publication WO 2008/135975 A2 relatesto a communication network, wherein the receiver wakes up for listeningto the preamble of a data packet and goes to sleep again, if no validpreamble is received.

U.S. Patent Application Publication 2006/0067550 A1 relates to a hearingaid system comprising at least three hearing aids between which awireless communication network is established using the Bluetoothstandard, wherein one of the hearing aids is used for receiving signalsfrom another one of the hearing aids, amplifying the signals andforwarding it to the third hearing aid.

U.S. Patent Application Publication US 2007/0086601 A1 relates to asystem comprising a transmission unit with a microphone for transmittinga speaker's voice to a plurality of hearing aids via a wireless digitallink, which may be unidirectional or bi-directional and which may beused for transmitting both audio data and control data to the hearingaids.

U.S. Pat. No. 7,529,565 B2 relates to a hearing aid comprising atransceiver for communication with an external device, wherein awireless communication protocol including a transmission protocol, linkprotocol, extended protocol, data protocol and audio protocol is used.The transmission protocol is adapted to control transceiver operationsto provide half duplex communications over a single channel, and thelink protocol is adapted to implement a packet transmission process toaccount for frame collisions on the channel.

U.S. Pat. No. 7,606,291 B2 relates to a two-way push-to-talk radiodevice using frequency hopping.

European Patent Application EP 1 560 383 A2 relates to a Bluetoothsystem, wherein the slave device, in a park mode or in a sniff mode,periodically wakes up to listen to transmission from the master and tore-synchronize its clock offset.

SUMMARY OF THE INVENTION

It is an object of the invention to provide for a sound transmissionsystem employing a digital audio link which has relatively low powerrequirement and which is particularly well-suited for a plurality ofreceiver units. It is also an object of the invention to provide for acorresponding sound transmission method.

According to the invention, these objects are achieved by a soundtransmission system and a sound transmission method as described herein.

The invention is beneficial in that, by using a link protocol whereinthe same audio packet is to be transmitted at least twice in the sameframe and wherein the frames are structured for unidirectional broadcasttransmission of audio data packets, without individually addressing thereceiver units (i.e., the frames do not include a receiver address) andwithout expecting any acknowledgement messages from the receivers, powerconsumption at the receiver side is kept low and a large number ofreceiver units can be used in the same network with the transmissionunit.

Preferably, the same audio packet is transmitted at least twice insubsequent slots. Preferably, the receiver units use the first verified,i.e., correctly received, copy/version of each data packet as the signalto be supplied to the stimulation means, while not using the audio dataof the other copies of the data packet. Usually, in the first slot ofeach frame, a beacon packet is to be transmitted which containsinformation for hopping frequency synchronization.

In order to further reduce power consumption, each receiver sleeps atleast during times when no data packets are to be expected and wakes upa given guard time before expected arrival of an audio packet differentto the previous audio packet. If no start frame delimiter has beenreceived or if the previous audio packet could not be verified, thereceiver wakes up a given guard time period before expected arrival ofthe repetition of the previous audio packet. If a start frame delimiterhas been received, the receiver goes to sleep again after a giventimeout period after the expected end of transmission of the audiopacket; if no start frame delimiter has been received, the receiver goesto sleep again after a given timeout period after the expected end oftransmission of the start frame delimiter of the audio packet; therebyfurther power consumption reduction can be achieved in case of missingpackets. Preferably, the start frame delimiter is a 5 bytes code buildfrom the 4 byte unique ID of the network master. This 5 byte code iscalled the network address, being unique for each network.

In order to achieve further power consumption reduction, each receivermay wake up a given guard time period before expected arrival of thebeacon packet of only certain ones of the frames, while sleeping duringexpected transmission of the beacon packet of the other frames. Inparticular, the receiver may make up only for beacon packets of frameshaving a sequence number which fulfills a given condition with regard tothe address of the respective receiver unit, so that the transmissionunit may send a message to that specific receiver unit by including themessage into the beacon packet of a frame having an appropriate sequencenumber. In addition, each receiver may wake up for the beacon packet offrames having a sequence number fulfilling a certain global condition(for example, every tenth frame), in order to have all receiversperiodically listen to the same beacon packet.

These and further objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionwhen taken in connection with the accompanying drawings which, forpurposes of illustration only, show several embodiments in accordancewith the present invention

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of audio components which can be used with asystem according to the invention;

FIG. 2 is a schematic view of a use of a first example of a systemaccording to the invention;

FIG. 3 is a schematic view of a use of a second example of a systemaccording to the invention;

FIG. 4 is a schematic view of a use of a third example of a systemaccording to the invention;

FIG. 5 is a schematic view of a use of a fourth example of a systemaccording to the invention;

FIG. 6 is a schematic block diagram of an example of a system accordingto the invention;

FIG. 7 is a more detailed example of the audio signal path in thetransmission unit of the system of FIG. 6;

FIG. 8 is a more detailed block diagram of an example of the receiverunit of the system of FIG. 6;

FIG. 9 is an example of the TDMA frame structure of the signals of thedigital audio link used in a system according to the invention;

FIG. 10 is an illustration of an example of the protocol of the digitalaudio link used in a system according to the invention in the connectedstate;

FIG. 11 is an illustration of an example of the protocol of the digitalaudio link used in an example of an assistive listening application withseveral companion microphones of a system according to the invention;

FIG. 12 is an illustration of an example of the protocol of the digitalaudio link used in an example of an assistive listening application withseveral receivers of a system according to the invention;

FIG. 13 is an illustration of an example of how a receiver unit in asystem according to the invention listens to the signals transmitted viathe digital audio link;

FIG. 14 is an illustration of an example of a frequency-hopping schemeused in a system according to the invention;

FIG. 15 is an illustration of the communication in a system according tothe invention during synchronization of the digital link;

FIG. 16 is an illustration of antenna diversity in a system according tothe invention; and

FIG. 17 is a further illustration of an example of a packet leveldiversity scheme used in a system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system for providing hearingassistance to at least one user, wherein audio signals are transmitted,by using a transmission unit comprising a digital transmitter, from anaudio signal source via a wireless digital audio link to at least onereceiver unit, from where the audio signals are supplied to means forstimulating the hearing of the user, typically a loudspeaker.

As shown in FIG. 1, the device used on the transmission side may be, forexample, a wireless microphone used by a speaker in a room for anaudience; an audio transmitter having an integrated or a cable-connectedmicrophone which are used by teachers in a classroom forhearing-impaired pupils/students; an acoustic alarm system, like a doorbell, a fire alarm or a baby monitor; an audio or video player; atelevision device; a telephone device; a gateway to audio sources like amobile phone, music player; etc. The transmission devices includebody-worn devices as well as fixed devices. The devices on the receiverside include headphones, all kinds of hearing aids, ear pieces, such asfor prompting devices in studio applications or for covert communicationsystems, and loudspeaker systems. The receiver devices may be forhearing-impaired persons or for normal-hearing persons. Also on thereceiver side, a gateway could be used which relays audio signalreceived via a digital link to another device comprising the stimulationmeans.

The system may include a plurality of devices on the transmission sideand a plurality of devices on the receiver side, for implementing anetwork architecture, usually in a master-slave topology.

The transmission unit typically comprises or is connected to amicrophone for capturing audio signals, which is typically worn by auser, with the voice of the user being transmitted via the wirelessaudio link to the receiver unit.

The receiver unit typically is connected to a hearing aid via an audioshoe or is integrated within a hearing aid.

Usually, in addition to the audio signals, control data is transmittedbi-directionally between the transmission unit and the receiver unit.Such control data may include, for example, volume control or a queryregarding the status of the receiver unit or the device connected to thereceiver unit (for example, battery state and parameter settings).

In FIG. 2, a typical use case is shown schematically, wherein abody-worn transmission unit 10 comprising a microphone 17 is used by ateacher 11 in a classroom for transmitting audio signals correspondingto the teacher's voice via a digital link 12 to a plurality of receiverunits 14, which are integrated within or connected to hearing aids 16worn by hearing-impaired pupils/students 13. The digital link 12 is alsoused to exchange control data between the transmission unit 10 and thereceiver units 14. Typically, the transmission unit 10 is used in abroadcast mode, i.e., the same signals are sent to all receiver units14.

Another typical use case is shown in FIG. 3, wherein a transmissiondevice 10 having an integrated microphone is used for capturing thevoice of a person 11 speaking to a hearing-impaired person 13 wearingreceiver units 14 connected to or integrated within a hearing aid 16.The captured audio signals are transmitted from device 10 via thedigital link 12 to the receiver units 14.

A modification of the use case of FIG. 3 is shown in FIG. 4, wherein thetransmission device 10 is used as a relay for relaying audio signalsreceived from a remote transmission unit 110 to the receiver units 14 ofthe hearing-impaired person 13. The remote transmission unit 110 is wornby a speaker 11 and comprises a microphone for capturing the voice ofthe speaker 11, thereby acting as a companion microphone.

According to a variant of the embodiments shown in FIGS. 2 to 4, thereceiver units 14 could be designed as a neck-worn device comprising atransmitter for transmitting the received audio signals via an inductivelink to an ear-worn device, such as a hearing aid.

The transmission units 10, 110 may comprise an audio input for aconnection to an audio device, such as a mobile phone, a FM radio, amusic player, a telephone or a TV device, as an external audio signalsource.

In FIG. 5, a use case is schematically shown which is similar to thatshown in FIG. 2 in that a teacher 11 in a classroom uses a body-worntransmission unit 10 comprising a microphone 17 for transmitting audiosignals corresponding to the teacher's voice via the digital audio link12 to a receiver unit 14 for reproducing the teacher's voice to students13. However, unlike in the case of FIG. 2, the receiver unit 14 is notworn by the respective student 13, but rather is connected to orintegrated within an audience loudspeaker system 18 arranged in theclassroom.

In each of such use cases, the transmission unit 10 usually comprises anaudio signal processing unit (not shown in FIGS. 2 to 5) for processingthe audio signals captured by the microphone prior to being transmitted.

A schematic block diagram of an example of a hearing assistance systemaccording to the invention is shown in FIG. 6. The system comprises atransmission unit 10 and at least one digital receiver unit 14.

The transmission unit 10 comprises a microphone arrangement 17 forcapturing a speaker's voice, which may be integrated within the housingof the transmission unit 10 or which may be connected to it via a cable.The transmission unit 10 also may include an audio signal input 19 whichserves to connect an external audio signal source 20, such as a mobilephone, an FM radio, a music player, a telephone or a TV device, to thetransmission unit 10.

The audio signals captured by the microphone arrangement 17 and/or theaudio signals optionally received from the external audio signal source20 are supplied to a digital signal processor (DSP) 22 which iscontrolled by a microcontroller 24 and which acts as an audio signalprocessing unit which applies, for example, a gain model to the capturedaudio signals.

In addition, the DSP 22 may serve to analyze the captured audio signalsand to generate control data (control commands) according to the resultof the analysis of the captured audio signals. The processed audiosignals and the control data/commands are supplied to a digitaltransmitter 28, which is likewise controlled by the microcontroller 24.

The digital transmitter 28 transmits the modulated signals via anantenna 36 to an antenna arrangement 38 of the digital receiver unit 14,thereby establishing a digital link 12. For implementing packet leveldiversity on the transmitter side, the transmission unit 10 may comprisea second antenna 30 which is spaced apart from the (first) antenna 36,typically at least one or several wavelengths of the carrier frequency.

In practice, both the digital transmitter 28 and the digital receiverunit 14 are designed as transceivers, so that the digital transceiver 28can also receive control data and commands sent from the digitalreceiver unit 14.

The transceiver 28 also may be used for receiving audio signals from anexternal audio source 25, such as a remote microphone used as acompanion microphone, via a wireless digital audio link 27, with thereceived audio signals being supplied to the DSP 22 for retransmissionby the transceiver 28. Hence, in this case, the transmission unit 10serves to relay audio signals from the external audio source to thereceiver unit 14 (see examples of FIGS. 4 and 11). Alternatively, thetransmission unit 10 may include a separate receiver (not shown in theFIGS. 6 and 7) for receiving the audio signals from the external audiosource; in this case the link 27 would be independent from the link 12and thus also could be analog.

The microcontroller 24 is responsible for management of all transmittercomponents and may implement the wireless communication protocol, inparticular for the digital link 12.

The digital receiver unit 14 comprises or is connected to a loudspeaker42 or another means for stimulating a user's hearing. Typically, thereceiver unit 14 is an ear-worn device which is integrated into orconnected to a hearing aid comprising the speaker 42. The control datatransmitted in parallel to the audio signals may serve to controloperation of the receiver unit 14 according to the presently prevailingauditory scene as detected by the DSP 22 from the audio signal capturedby the microphone arrangement 17.

In FIG. 7, an example of the audio signal path in the transmission unit10 is shown in more detail.

The microphone arrangement 17 of the transmission unit 10 comprises twospaced apart microphones 17A, 17B for capturing audio signals which aresupplied to an acoustic beam-former unit 44 which generates an outputsignal supplied to a gain model unit 46. The output of the beam-formerunit 44 is also supplied to a voice activity detector (VAD) unit 48which serves to detect whether the speaker is presently speaking or notand which generates a corresponding status output signal. The output ofat least one of the microphones 17A, 17B is also supplied to an ambientnoise estimation unit 50 which serves to estimate the ambient noiselevel and which generates a corresponding output signal. The outputsignals of the units 48, 50 and the processed audio signals from thegain model 46 are supplied to a unit 56 which serves to generate acorresponding digital signal comprising the audio signals and thecontrol data which is supplied to the digital transceiver 28. Theexternal audio signals optionally received via the audio input 19 and/orthe transceiver 28 may be supplied to the gain model 46.

The units 44, 46, 48, 50 and 56 may be functionally realized by the DSP22 (see dashed line surrounding these units in FIG. 7).

As already mentioned with regard to FIG. 6, the transmission unit 10 maycomprise a second antenna 30 which is spaced apart from the firstantenna 36. Such a dual antenna arrangement may be used to transmit anaudio data packet via the first antenna 36 and to subsequently transmita repeated copy of the same audio data packet via the second antenna 30,as will be explained in more detail with regard to FIGS. 9 and 10.

A more detailed example of the digital receiver unit 14 is shown in FIG.8, according to which the antenna arrangement 38 may comprises twoseparate antennas 38A, 38B, wherein the first antenna 38A is connectedto a first digital receiver 61A including a demodulator 58A and a buffer59A and the second antenna 38B is connected to a second digital receiver61B including a demodulator 58B and a buffer 59B. The two parallelreceivers may be utilized for a applying a packet level diversity schemeto the signals received via the digital link 12, as will be explainedbelow in more detail with regard to FIGS. 16 and 17.

The antennas 38A, 38B receive the signals that are transmitted via thedigital link 12 and the received signals are demodulated by thedemodulators 58A, 58B in the digital radio receivers 61A, 61B. Thedemodulated signals are supplied via the buffers 59A, 59B to a DSP 74acting as processing unit which separates the signals into the audiosignals and the control data and which is provided for advancedprocessing, e.g., equalization, of the audio signals according to theinformation provided by the control data. The processed audio signals,after digital-to-analog conversion, are supplied to a variable gainamplifier 162 which serves to amplify the audio signals by applying again controlled by the control data received via the digital link 12.The amplified audio signals are supplied to a hearing aid 64.Alternatively, the variable gain amplifier may be realized in thedigital domain by using a PWM modulator taking over the role of theD/A-converter and the power amplifier. The receiver unit 14 alsoincludes a memory 76 for the DSP 74.

Rather than supplying the audio signals amplified by the variable gainamplifier 162 to the audio input of a hearing aid 64, the receiver unit14 may include a power amplifier 78 which may be controlled by a manualvolume control 80 and which supplies power amplified audio signals to aloudspeaker 82 which may be an ear-worn element integrated within orconnected to the receiver unit 14. Volume control also could be achievedremotely from the transmission unit 10 by transmitting correspondingcontrol commands to the receiver unit 14.

Alternatively, rather than being ear-worn components, the receiver unit14 could be located somewhere in a room in order to supply audio signalsto loudspeakers 82 installed in the same room, whereby a speechenhancement system for an audience can be realized (as indicated bydashed lines in FIG. 8).

Another alternative implementation of the receiver maybe a neck-worndevice having a transmitter 84 for transmitting the received signals viawith an magnetic induction link 86 (analog or digital) to the hearingaid 64 (as indicated by dotted lines in FIG. 8).

In general, the role of the microcontroller 24 could also be taken overby the DSP 22. Also, signal transmission could be limited to a pureaudio signal, without adding control and command data.

Details of the protocol of the digital link 12 will be discussed byreference to FIGS. 9 to 13. Typical carrier frequencies for the digitallink 12 are 865 MHz, 915 MHz and 2.45 GHz, wherein the latter band ispreferred. Examples of the digital modulation scheme are PSK/FSK, ASK orcombined amplitude and phase modulations such as QPSK, and variationsthereof (for example GFSK).

The preferred codec used for encoding the audio data is ADPCM (AdaptiveDifferential Pulse-Code Modulation).

In addition, packet loss concealment (PLC) may be used in the receiverunit. PLC is a technique which is used to mitigate the impact of lostaudio packets in a communication system, wherein typically thepreviously decoded samples are used to reconstruct the missing signalusing techniques such as wave form extrapolation, pitch synchronousperiod repetition and adaptive muting.

Preferably, data transmission occurs in the form of TDMA (Time DivisionMultiple Access) frames comprising a plurality of time slots (forexample, 10), wherein one data packet may be transmitted in each slot.In FIG. 9, an example is shown wherein the TDMA frame has a length of 4ms and is divided into 10 time slots of 400 μs, with each data packethaving a length of 160 μs.

As will be explained by reference to FIGS. 14 and 15 below, preferably aslow frequency hopping scheme is used, wherein each slot is transmittedat a different frequency according to a frequency hopping sequencecalculated by a given algorithm in the same manner by the transmitterunit 10 and the receiver units 14, wherein the frequency sequence is apseudo-random sequence depending on the number of the present TDMA frame(sequence number), a constant odd number defining the hopping sequence(hopping sequence ID) and the frequency of the last slot of the previousframe.

The first slot of each TDMA frame (slot 0 in FIG. 9) is allocated to theperiodic transmission of a beacon packet which contains the sequencenumber numbering the TDMA frame and other data necessary forsynchronizing the network, such as information relevant for the audiostream, such as description of the encoding format, description of theaudio content, gain parameter, surrounding noise level, etc.,information relevant for multi-talker network operation, and optionallycontrol data for all or a specific one of the receiver units.

The second slot (slot 1 in FIG. 9) may be allocated to the reception ofresponse data from slave devices (usually the receiver units) of thenetwork, whereby the slave devices can respond to requests from themaster device through the beacon packet. At least some of the otherslots are allocated to the transmission of audio data packets, whereineach audio data packet is repeated at least once, typically insubsequent slots. In the example shown in FIGS. 9 and 10, slots 3, 4 and5 are used for three-fold transmission of a single audio data packet.The master device does not expect any acknowledgement from the slavesdevices (receiver units), i.e., repetition of the audio data packets isperformed in any case, irrespective of whether the receiver unit hascorrectly received the first audio data packet (which, in the example ofFIGS. 9 and 10, is transmitted in slot 3) or not. Also, the receiverunits are not individually addressed by sending a device ID, i.e., thesame signals are sent to all receiver units (broadcast mode).

Rather than allocating separate slots to the beacon packet and theresponse of the slaves, the beacon packet and the response data may bemultiplexed on the same slot, for example, slot 0.

The audio data maybe compressed in the transmission unit 10 prior tobeing transmitted.

If the transmission unit 10 comprises two antennas 30, 36, packet leveldiversity with regard to the audio data packets may be realized on thetransmitter side by transmitting each one of the copies of the sameaudio data packet alternately via a different one of the antennas 30,36. For example, the first copy of the audio data packet (which, in theexample of FIGS. 9 and 10, is transmitted in slot #3, may be transmittedvia the antenna 36, whereas the second copy (in slot #4) may betransmitted via the antenna 30, while the third copy (in slot #5) may betransmitted again via the antenna 36. If for example, at the position ofthe antenna 36 multi-path fading occurs with regard to the antenna ofthe receiver unit 14, it is unlikely that multi-path fading likewiseoccurs at the position of the antenna 30, so at least one copy will betransmitted/received without fading.

In FIG. 11, an example of a more complex slot allocation scheme isshown, wherein, as in the example of FIGS. 9 and 10, slot 0 is allocatedto the beacon packet from the master device and slot 1 is allocated toresponse data packets. However, in the example of FIG. 11, each audiodata packet is repeated only once and a transmission unit 10 is used asa relay/gateway between three remote transmission units 110A, 110B and110C acting as companion microphones and two receiver units 14A, 14B.Slots 2 and 3, slots 4 and 5 and slots 6 and 7 are used for transmissionof audio data from the first external transmission unit 110A, the secondexternal transmission unit 110B and the third external transmission unit110C, respectively, towards the relay/gateway transmission unit 10, andslots 8 and 9 are allocated to transmission of audio data packets fromthe relay/gateway transmission unit 10 to the receiver units 14A, 14B.The beacon packet in slot 0 is sent from the unit 10 acting as themaster to all slaves, i.e. the units 110A, 110B, 110C, 14A and 14B. Thebeacon packet and the response packet can also be time-multiplexed onthe same slot 0 (e.g., even numbered TDMA frames for beacon packets, oddnumbered TDMA frames for response packets).

Usually, in a synchronized state, each slave listens only to specificbeacon packets (the beacon packets are needed primarily forsynchronization), namely those beacon packets for which the sequencenumber and the ID address of the respective slave device fulfills acertain condition, whereby power can be saved. When the master devicewishes to send a message to a specific one of the slave devices, themessage is put into the beacon packet of a frame having a sequencenumber for which the beacon listening condition is fulfilled for therespective slave device. This is illustrated in FIG. 12, wherein thefirst receiver unit 14A listens only to the beacon packets sent by thetransmission unit 10 in the frames number 1, 5, etc., the secondreceiver unit 14B listens only to the beacon packets sent by thetransmission unit 10 in the frames number 2, 6, etc., and the thirdreceiver unit 14C listens only to the beacon packet sent by thetransmission unit 10 in the frames number 3, 7, etc.

Periodically, all slave devices listen at the same time to the beaconpacket, for example, to every tenth beacon packet (not shown in FIG.12).

Each audio data packet comprises a start frame delimiter (SFD), audiodata and a frame check sequence, such as CRC (Cyclic Redundancy Check)bits. Preferably, the start frame delimiter is a 5 bytes code built fromthe 4 byte unique ID of the network master. This 5 byte code is calledthe network address, being unique for each network.

In order to save power, the receivers 61A, 61B in the receiver unit 14are operated in a duty cycling mode, wherein each receiver wakes upshortly before the expected arrival of an audio packet. If the receiveris able to verify (by using the CRC at the end of the data packet), thereceiver goes to sleep until shortly before the expected arrival of anew audio data packet (the receiver sleeps during the repetitions of thesame audio data packet), which, in the example of FIGS. 9 and 10, wouldbe the first audio data packet in the next frame. If the receiverdetermines, by using the CRC, that the audio data packet has not beencorrectly received, the receiver switches to the next frequency in thehopping sequence and waits for the repetition of the same audio datapacket (in the example of FIGS. 9 and 10, the receiver then would listento slot 4 as shown in FIG. 10, wherein in the third frame transmissionof the packet in slot 3 fails).

In order to further reduce power consumption of the receiver, thereceiver goes to sleep shortly after the expected end of the SFD, if thereceiver determines, from the missing SFD, that the packet is missing orhas been lost. The receiver then will wake up again shortly before theexpected arrival of the next audio data packet (i.e., thecopy/repetition of the missing packet).

An example of duty cycling operation of the receiver is shown in FIG.13, wherein the duration of each data packet is 160 μs and wherein theguard time (i.e., the time period by which the receiver wakes up earlierthan the expected arrival time of the audio packet) is 20 μs and thetimeout period (i.e., the time period for which the receiver waits afterthe expected end of transmission of the SFD and CRC, respectively)likewise is 20 μs. It can be seen from FIG. 13 that, by sending thereceiver to sleep after timing out of SFD transmission (when no SFD hasbeen received), the power consumption can be reduced to about half ofthe value when the receiver is sent to sleep after timeout of CRCtransmission.

As already mentioned above, a pseudo-random frequency hopping scheme isused for data transmission. As illustrated in FIG. 14, for calculatingthe frequency-hopping sequence, an algorithm is used which has, as inputparameters, the frequency f_(p) used for the last slot of the previousframe, the hopping sequence ID (HSID) and the sequence number s of thepresent frame. The algorithm uses a linear congruent generator (LCG)which outputs the frequency for each slot of the frame based on thesethree input parameters. An example of the computation of f_(i),iϵ{0;9},based on the three parameters HSID, s and f_(p) are given below:

Initialisation of Constantsc=HSIDm=2¹⁶r=s

Computation of f₀ Based on f_(p)r=mod(17·r+c,m){circumflex over (r)}=(19·r)/2¹⁶f₀=mod(f_(p)+11+{circumflex over (r)},40)

Computation of f₁, f₂, . . . , f₉ for Each f_(i), iϵ{1:9}r=mod(17·r+c,m){circumflex over (r)}=(19·r)/2¹⁶f_(i)=mod(f_(i-1)+11+{circumflex over (r)},40)

The information necessary to compute the frequency-hopping sequence forthe present frame is transmitted in the beacon packet in the first slotof the frame from the master device to the slave devices. The hoppingsequence ID is not included in the beacon packet, but rather istransmitted in a pairing phase to the slave devices and is stored ineach slave device. Once synchronized to the master device, the slavedevices increment the sequence number automatically to calculate thefrequency at which the beacon packet of the next frame is to bereceived.

The Hopping Sequence ID is chosen as an odd number between 1 and 65535 .. . . This number is chosen randomly by the network master (relay unit15) and transmitted to the network slaves (transmission units 10 andreceiver units 14) during pairing. This odd number is used as theadditive term of the LCG. By selecting the hopping sequence ID randomly,it is provided that the hopping sequence is likely to be unique to thepresent network, so that there is only low cross-correlation with thehopping sequence of another network which may exist, for example, in thesame building. In the unlikely event that two networks select the samehopping sequence ID and disturb each other, a new pairing process in oneof the networks is likely to result in a different hopping sequence ID.The use of the frequency of the last slot of the previous frame in thehopping sequence algorithm ensures that there is always a minimumdistance between two subsequent slots, namely also between the last slotof the previous frame and the first slot of the present frame.

Preferably, the frequency-hopping scheme is an adaptivefrequency-hopping scheme, wherein packet error rate measurements aremade for the used frequencies and wherein the master device may decide,based on such measurements, that a sub-set of the available frequenciesshould be declared as “bad frequencies” and should not be used anylonger. If then, the frequency computation algorithm selects one of thebad frequencies, a frequency is pseudo-randomly selected instead, from aset of frequencies composed of all “good frequencies” at the exceptionof the good frequency used in the preceding slot. Removing the frequencyused in the preceding slot from the set of potential replacementfrequencies presents the advantage of avoiding the possibility of usingthe same frequency twice in consecutive slots.

FIG. 15 illustrates how synchronization between the master device (forexample, the transmission unit 10) and the slave devices (for example,one of the receiver units 14) may be achieved.

The synchronization is passive in the sense that there is no feedbackfrom the slave device to the master device during synchronization.Usually, the master device, e.g., the transmission unit 10, does notdistinguish whether a certain one of the slaves, e.g., the receiverunits 14, is in still a synchronization mode or already in asynchronized mode, so that the transmission operation of the master isalways the same, i.e., the same algorithm for determining the hoppingsequences is used and the same protocol is used, e.g., beacon packet inthe first slot, audio data packets in some of the other slots (as longas audio signals are generated in/supplied to the transmission unit; theaudio data packets are not shown in FIG. 15).

Thus, the master device transmits a beacon packet in regular intervals,namely in the first slot of each TDMA frame (according to the example, abeacon packet is sent every 4 ms). The frequency at which the respectivebeacon packet is sent is calculated according to the same pseudo-randomhopping-sequence algorithm which is used for transmitting audio packetsin the synchronized state. The hopping sequence is long in the sensethat it is much longer/larger than the number of frequency channels (forexample, a sequence of the length 100 is likely to show a badcorrelation with another sequence of the length 100, depending on thetime shift). The slave device listens periodically for the first beaconpacket for synchronization, i.e., it is operated in a duty cycling mode.The listening time period is longer than the duration of the beaconpacket. Each listening period is done at a different frequency; forexample, the first listening period may at the lowest frequency of theavailable band (i.e., the receiver listens in the lowest one of thefrequency channels), and then the listening frequency is increased foreach subsequent listening period (thereby going systematically throughall frequency channels). After each listening period, the receiver goesback to sleep. The periodicity of the listening periods is chosen closeto the beacon packet transmission periodicity (i.e., the frame length),but it is not exactly equal, in order to have a drift between the beaconpacket transmission phase and the listening phase. Due to this drift,the listening phase is periodically in phase with the transmission ofthe beacon packet for a certain duration. When the beacon packet istransmitted at the same frequency as the one used presently forlistening, synchronization is achieved and the receiver switches intothe synchronized mode/state, wherein it can calculate the hoppingsequence presently used by the transmission unit from the informationincluded in the received beacon packet (i.e., the frame sequence number)and the hopping sequence ID stored in the receiver unit from the pairingphase. A more detailed explanation of this synchronization method isgiven below.

When a receiver is in the synchronization phase, it listens periodicallywith period T_(ListenPeriod) for a duration T_(ListenDuration) at agiven frequency and then goes back to sleep. The frequency is changedfor each listening phase starting with frequency number 0, andincrementing up to e.g., frequency 39. The beacon is transmitted on anyof the 40 frequencies, following the pseudo-random frequency selection.

The period T_(ListenPeriod) is chosen to be close to the beacontransmission period T_(BeaconPeriod) but not to be exactly equal. Thedifference ΔT=|T_(ListenPeriod)−T_(BeaconPeriod) causes a drift betweenthe beacon packet transmission phase and the listening phase. Due tothis drift, the listening phase is periodically in phase with thetransmission of the beacon packet for a certain duration. If the beaconpacket is transmitted at the same frequency as the one used forlistening, synchronization is achieved. This mechanism is illustrated inFIG. 15.

The values of parameters T_(ListenPeriod), T_(ListenDuration) are to bechosen based on the beacon packet period T_(BeaconPeriod) and on thebeacon packet duration T_(BeaconDuration), as a trade-off between thesynchronization delay and the synchronization power consumption.

With T_(ListenPeriod)=T_(BeaconPeriod)(1+θ), and ΔT=θT_(BeaconPeriod) isthe shift in phase of the listening activity for every transmission ofthe beacon packet.

T_(ListenDuration) must be larger than T_(BeaconDuration) such that itis possible to receive a beacon packet. An additional margin ΔT isrequired such that the listen window is open for the duration of thebeacon packet transmission, given the fact that the listen window isdrifting compared to the transmission window. A larger margin than ΔTgives the opportunity for the reception of more than one beacon packetin a given transmission window.

The time interval between two in-phase periods will be

$\begin{matrix}{T_{InPhasePeriod} = \frac{T_{{BeaconPeriod}\;}T_{ListenPeriod}}{\Delta\; T}} \\{= \frac{T_{{BeaconPeriod}\;}T_{ListenPeriod}}{\theta\; T_{BeaconPeriod}}} \\{= \frac{T_{ListenPeriod}}{\theta}} \\{= {T_{BeaconPeriod}\frac{1 + \theta}{\theta}}}\end{matrix}$

When the transmission and listening intervals are in phase, there willbe enough time for a limited number of transmission trials, until thewindows are out of phase again. The number of possible trials is givenby

${N_{TrialInPhase} = \left\lfloor \frac{T_{ListenDuration} - T_{BeaconDuration}}{\Delta\; T} \right\rfloor},$

where └ ┘ means rounded to the nearest integer in a direction towardszero.

The average synchronization delay can then be computed with

${{\overset{\_}{T}}_{Synchronization} = \frac{T_{InPhasePeriod}/N_{TrialsInPhase}}{1/N_{Channels}}},$when N_(Channels)=40, θ=0.01, T_(BeaconPeriod)=4 ms,T_(ListenDuration)=600 μs, T _(Synchronization)=1.6 s and the duty cyclewill be, in this case,

$\eta = {\frac{T_{ListenDuration}}{T_{ListenPeriod}} = {\frac{600}{4000} = {15{\%.}}}}$

A further refinement can be obtained if a transmission unit has tworadios, i.e., transmitters/transceivers. In such case, the two radiosmay be used to transmit the beacon messages in an inter-leaved manner,or in parallel and at different frequencies. This method would reducethe synchronization time required at the receiver side.

As illustrated in FIG. 16, by using two spaced-apart antennas 38A, 38Bmulti-path fading resulting from destructive interference betweenseveral copies of the same signal travelling due to multiple reflectionsalong different signal paths with different lengths (for example, directsignal and signal reflected once), can be mitigated, since theinterference conditions are different at different positions, i.e., ifdestructive interference occurs at the position of one of the antennas,it is likely that no destructive interference occurs at the position ofthe other antenna. In other words, if the two antennas are sufficientlyspaced apart, the fading events are uncorrelated on both antennas.

The present invention may utilize this effect by applying a packet leveldiversity scheme in the receiver unit. When a data packet has beenreceived by the receiver 58A, it will be verified by using the CRC andit will be buffered in the buffer 59A. In addition, an interrupt requestis sent from the receiver 59A to the processing unit 74, in order toindicate that a packet has been received. The other receiver 58B acts inparallel accordingly: when it receives a data packet, it verifies thedata packet and buffers it in the buffer 59B and sends an interruptrequest to the processing unit 74.

When the processing unit 74 receives such an interrupt request, it readsthe data packet from one of the two buffers 59A, 59B (usually, there isa default setting as to from which one of the buffers the processingunit 74 will try to read the data packet first) and flushes the otherone of the buffers 59A, 59B, if the data packet was obtained correctly(rather than using interrupt requests, the respective buffer 59A, 59Bcould be checked at the end of the last reception slot; i.e., thereceivers could operated via polling rather than via interrupts).However, if it is not possible to read the data packet from the defaultone of the buffers (usually because the respective antenna 38A, 38Bsuffered from severe multi-path fading at the reception time), theprocessing unit 74 tries to read the data packet from the other one ofthe buffers and, if it is successful in reading the data packet, itflushes the buffer of the other.

An example of this method is illustrated in FIG. 17, wherein it isassumed that the third transmission of the data packet “A” from thetransmission unit 10 fails at the antenna 38A allocated to the receiver58A so that, in this case, the processing unit 74 reads the data packetfrom the buffer 59B of the receiver 58B rather than from the buffer 59Aof the receiver 58A (which, in the example, is the default receiver).Typically, such packet level diversity is applied not only to the audiodata packets, but also to the other data packets, such as the beaconpacket.

However, it is noted that such packet level diversity is not applicableto ear level receiver units since, due to the small size of ear levelreceiver units, there is usually not enough space for the requiredspatial separation of the two antennas required for the above-describedpacket level diversity scheme.

While various embodiments in accordance with the present invention havebeen shown and described, it is understood that the invention is notlimited thereto, and is susceptible to numerous changes andmodifications as known to those skilled in the art. Therefore, thisinvention is not limited to the details shown and described herein, andincludes all such changes and modifications as encompassed by the scopeof the appended claims.

What is claimed is:
 1. A system for providing sound to at least oneuser, comprising: at least one audio signal source for providing audiosignals; a transmission unit comprising a digital transmitter forapplying a digital modulation scheme in order configured to transmit theaudio signals as data packets from the audio signal source via awireless digital audio link; at least one receiver unit for reception ofconfigured to receive audio signals from the transmission unit via thedigital audio link, comprising at least one digital receiver; means forstimulating hearing of at least one user according to audio signalssupplied from the receiver unit; wherein the transmission unit isadapted configured to transmit each data packet in a separate slot of aTDMA frame at a different frequency according to a frequency hoppingsequence, wherein, in at least some of the slots, the audio signals areto be transmitted as audio data packets, wherein a same audio packet isto be transmitted at least twice in a same TDMA frame, without expectingacknowledgement messages from the at least one receiver unit, whereinthe TDMA frames are structured for unidirectional broadcast transmissionof the audio data packets, without individually addressing the at leastone receiver unit, and wherein each audio data packet comprises a startframe delimiter, audio data and a frame check sequence, and wherein eachdigital receiver of the at least one receiver unit is adapted configuredto verify each received data packet by using the frame check sequenceand to use the audio data of the first verified version of each datapacket as the signal to be supplied to the stimulation means while notusing the audio data of other versions.
 2. The system of claim 1,wherein the same audio packet is to be transmitted at least twice insubsequent slots.
 3. The system of claim 1, wherein each receiver of theat least one receiver unit is adapted to sleep at least during timeswhen no data packets are to be expected.
 4. The system of claim 3,wherein each receiver of the at least one receiver unit is adapted towake up a given guard time period before expected arrival of an audiopacket different from the previous audio packet.
 5. The system of claim4, wherein each receiver of the at least one receiver unit is adapted towake up at a given guard time period before expected arrival of therepetition of the previous audio packet, if a start frame delimiter hasnot been received or if the previous audio packet could not be verified.6. The system of claim 5, wherein each receiver of the at least onereceiver unit is adapted to go to sleep again after a given timeoutperiod after an expected end of transmission of the audio packet, if noaudio packet has been received.
 7. The system of claim 6, wherein eachreceiver is adapted to go to sleep again after a given timeout periodafter an expected end of transmission of the start frame delimiter ofthe audio packet, if a start frame delimiter has not been received. 8.The system of claim 1, wherein, in a first slot of each frame, a beaconpacket is to be transmitted which contains information for hoppingfrequency synchronization.
 9. A system for providing sound to at leastone user, the system comprising: at least onean audio signal source forproviding audio signals; a transmission unit comprising a digitaltransmitter for applying a digital modulation scheme in order configuredto transmit the audio signals as data packets from the audio signalsource via a wireless digital audio link; at least onea receiver unitconfigured for reception of audio signalsthe data packets from thetransmission unit via the digital audio link, comprising at leastonewherein the receiver unit includes a digital receiver; means forstimulating hearing of at least one providing at least portions of theaudio signals to a user according to audio signals supplied from thereceiver unit;, wherein the transmission unit is adapted to configuredto transmit each data packet in a separate slot of a TDMA time-slottedframe at a different frequency according to a frequency hoppingsequence, wherein, in at least some of the slots, the audio signals datapackets are to be transmitted as audio data packets, and wherein a sameeach audio data packet is to be transmitted at least twice in a the sameTDMA time-slotted frame, without expecting acknowledgement messages fromthe at least one receiverunit, wherein, in a first slot of each frame, abeacon packet to be transmitted is provided which contains datanecessary for synchronizing the network, and wherein each receiver ofthe at least one receiver unit is adapted to wake up a given guard timeperiod before expected arrival of the beacon packet of certain ones ofthe frames, while sleeping during expected transmission of the beaconpacket of the other frames.
 10. The system of claim 9, wherein eachreceiver of the at least one the receiver unit is adapted to wake up ata given guard time period before expected arrival of beacon packets offrames having a sequence number which fulfills a given condition withregard to an address of the respective receiver unit having a sequencenumber which fulfills a given condition with regard to periodicity. 11.The system of claim 10, wherein the at least one receiver unit compriseis a first receiver unit and the system further comprises a plurality ofreceiver units, and wherein the transmission unit is adapted to send amessage to a specific one of the receiver units by including the messageinto the beacon packet of a frame having a sequence number for which thegiven condition is fulfilled with regard to an address of the arespective receiver unit.
 12. A system for providing sound to at leastone user, comprising: at least one audio signal source for providingaudio signals; a transmission unit comprising a digital transmitter forapplying a digital modulation scheme in order to transmit the audiosignals as data packets from the audio signal source via a wirelessdigital audio link; at least one receiver unit for reception of audiosignals from the transmission unit via the digital audio link,comprising at least one digital receiver; means for stimulating hearingof at least one user according to audio signals supplied from thereceiver unit; wherein the transmission unit is adapted to transmit eachdata packet in a separate slot of a TDMA frame at a different frequencyaccording to a frequency hopping sequence, wherein, in at least some ofthe slots, the audio signals are to be transmitted as audio datapackets, wherein a same audio packet is to be transmitted at least twicein a same TDMA frame, without expecting acknowledgement messages fromthe at least one receiver unit, wherein, in a first slot of each frame,a beacon packet to be transmitted is provided which contains datanecessary for synchronizing the network, and wherein the at least onereceiver unit comprise a plurality of receiver units, and wherein eachreceiver unit is adapted to wake up at a given guard time period beforeexpected arrival of the beacon packet of frames having a sequence numberfulfilling a certain global condition, in order to have all receiverunits periodically listen to a same beacon packet.
 13. The system ofclaim 8, wherein the transmission unit is adapted to receive in a secondslot of each frame a control data packet from the receiver unitsrequested by the transmission unit.
 14. The system of claim 8, wherein afirst slot of each frame is for multiplexing a beacon packet to be sentby the transmission unit and a control data packet to be received fromthe at least one receiver unit as requested by the transmission unit.15. A system for providing sound to at least one user, comprising: atleast one audio signal source for providing audio signals; atransmission unit comprising a digital transmitter for applying adigital modulation scheme in order to transmit the audio signals as datapackets from the audio signal source via a wireless digital audio link;at least one receiver unit for reception of audio signals from thetransmission unit via the digital audio link, comprising at least onedigital receiver; means for stimulating hearing of at least one useraccording to audio signals supplied from the receiver unit; wherein thetransmission unit is adapted to transmit each data packet in a separateslot of a TDMA frame at a different frequency according to a frequencyhopping sequence, wherein, in at least some of the slots, the audiosignals are to be transmitted as audio data packets, wherein a sameaudio packet is to be transmitted at least twice in a same TDMA frame,without expecting acknowledgement messages from the at least onereceiver unit, wherein, in a first slot of each frame, a beacon packetto be transmitted is provided which contains data necessary forsynchronizing the network, and wherein the at least one receiver unitcomprise a plurality of receiver units, and wherein each beacon packetincludes at least one item of information relevant for an audio streamfrom the group of items comprising a description of encoding format, adescription of audio content, a gain parameter, surrounding noise level;information relevant for multi-talker network operation, and controldata for all or a specific one of the receiver units.
 16. The system ofclaim 1, wherein the transmission unit is adapted to encode the audiodata using ADPCM.
 17. The system of claim 1, wherein the audio signalsource is a microphone arrangement integrated into or connected to thetransmission unit for capturing a speaker's voice.
 18. The system ofclaim 17, wherein the transmission unit comprises an audio signalprocessing unit for processing the audio signals captured by themicrophone arrangement prior to being transmitted.
 19. The system ofclaim 1, wherein the transmission unit is adapted to establish thedigital audio link at a carrier frequency in a 2.4 GHz ISM band.
 20. Thesystem of claim 1, wherein the transmission unit is adapted to beconnected to an external audio device from the group of external audiodevices comprising a mobile phone, an FM radio, a music player, atelephone and a TV device, as the audio signal source.
 21. The system ofclaim 1, wherein the transmission unit is adapted to be connected via adigital audio link to an external transmission unit comprising amicrophone for capturing a speaker's voice as the audio signal source.22. The system of claim 1, wherein the at least one receiver unitcomprise a plurality of receiver units, and wherein at least one of thereceiver units is connected to or integrated into an ear-worn device,comprising the stimulation means.
 23. The system of claim 1, wherein theat least one receiver unit comprise a plurality of receiver units, andwherein at least one of the receiver units is a neck-worn devicecomprising a transmitter for transmitting audio signals via an inductivelink to an ear-worn device, comprising the stimulation means.
 24. Thesystem of claim 1, wherein the at least one receiver unit comprise aplurality of receiver units, and wherein the at least one receiver unitis connected to or integrated within at least one audience loudspeakerserving as the stimulation means.
 25. A system for providing sound to atleast one user, comprising: at least one audio signal source forproviding audio signals; a transmission unit comprising a digitaltransmitter for applying a digital modulation scheme in order totransmit the audio signals as data packets from the audio signal sourcevia a wireless digital audio link; at least one receiver unit forreception of audio signals from the transmission unit via the digitalaudio link, comprising at least one digital receiver; means forstimulating hearing of at least one user according to audio signalssupplied from the receiver unit; wherein the transmission unit isadapted to transmit each data packet in a separate slot of a TDMA frameat a different frequency according to a frequency hopping sequence,wherein in at least some of the slots, the audio signals are to betransmitted as audio data packets, wherein a same audio packet is to betransmitted at least twice in a same TDMA frame, without expectingacknowledgement messages from the at least one receiver unit, whereinthe TDMA frames are structured for unidirectional broadcast transmissionof the audio data packets, without individually addressing the at leastone receiver unit, and wherein the at least one receiver unit comprise aplurality of receiver units, and wherein at least one of the receiverunits is a neck-worn device comprising a transmitter for transmittingaudio signals via an inductive link to an ear-worn device, comprisingthe stimulation means, and wherein the at least one receiver unitcomprise a plurality of receiver units, wherein each receiver unitcomprises at least two digital receivers, each being connected to acommon processing unit and an antenna and including a demodulator and abuffer, wherein each receiver is for receiving, verifying and bufferingeach of the audio data packets, wherein each receiver unit is adapted tosend an interrupt request from at least one of the receivers to theprocessing unit to indicate when a packet has been received, and whereinthe processing unit is adapted to read an audio data packet from abuffer of one of the receivers and, if the packet has been correctlyreceived by that receiver, to flush the buffer of the other receiversand, if the packet has not been correctly received by that receiver, toread the audio data packet from the buffer of another one of thereceivers.
 26. A method for providing sound, the method comprising:transmitting an audio signal as audio data packets from a transmissionunit via a wireless digital audio link; receiving, at a receiver unit,the audio data packets from the transmission unit via the digital audiolink, wherein the receiver unit includes a digital receiver, wherein thetransmission unit transmits each audio data packet twice in separateslots of a time-slotted frame at different frequencies according to afrequency hopping sequence, and wherein transmissions of the twicetransmitted audio data packets are executed unconditionally,irrespective of whether the receiver unit has correctly received one ofthe transmissions of the audio data packets.
 27. The method of claim 26,wherein each frame comprises a start frame delimiter, the two audio datapackets, and a frame check sequence, and wherein the digital receiver ofthe receiver unit verifies each received data packet by using a framecheck sequence.
 28. The method of claim 26, wherein the receiver unit iselectronically coupled to or integrated in a hearing aid, headphone, orearphone.
 29. A method for providing sound, the method comprising:receiving an audio signal; converting the audio signal to digital audiopackets; transmitting wirelessly the digital audio packets to a receiverunit, wherein the transmitting further includes transmitting a firstdigital audio packet on a first frequency in a first time slot of atime-slotted frame, wherein the transmitting further includestransmitting a second digital audio packet on a second frequency in asecond time slot of the time-slotted frame, and wherein the firstdigital audio packet and the second digital audio packet include thesame audio information and are transmitted in subsequent slots of thetime-slotted frame.
 30. The method of claim 29, the method furthercomprising: determining an expected arrival time when the first digitalaudio packet is expected to arrive; determining a time when the receiverunit will start listening by subtracting a guard period from theexpected arrival time; and in response to determining the time when thereceiver unit will start listening, waking up the receiver unit to startlistening at the time.
 31. The method of claim 29, the method furthercomprising: determining that the first digital audio packet has beencorrectly received; in response to determining the first digital audiopacket has been correctly received, sleeping during the transmission ofthe second digital audio packet.
 32. The method of claim 29, the methodfurther comprising: in response to determining the first digital audiopacket was not received, switching the receiver unit to receive thesecond digital audio packet on the second frequency and waiting for thesecond digital packet.
 33. The method of claim 29, wherein the receiverunit is electronically coupled to or integrated into a hearing aid orheadphone.
 34. The method of claim 29, wherein the time-slotted frameincludes a zero time slot, wherein the zero time slot includes a beaconpacket, and wherein the beacon packet includes data for synchronizing.35. A non-transitory computer-readable medium storing instructions,which when executed by a device, cause the device to perform operationsfor providing audio to multiple receivers, the operations comprising:receive an audio signal; convert the audio signal to digital audiopackets; transmit wirelessly the digital audio packets to a receiverunit, wherein the transmitting further includes transmitting a firstdigital audio packet on a first frequency in a first time slot of atime-slotted frame, wherein the transmitting further includestransmitting a second digital audio packet on a second frequency in asecond time slot of the time-slotted frame, and wherein the firstdigital audio packet and the second digital audio packet include thesame audio information and are transmitted in subsequent slots of thetime-slotted frame.
 36. The non-transitory computer readable medium ofclaim 35, the operations further comprising: determine an expectedarrival time when the first digital audio packet is expected to arrive;determine a time when the receiver unit will start listening bysubtracting a guard period from the expected arrival time; and inresponse to determining the time when the receiver unit will startlistening, wake up the receiver unit to start listening at the time. 37.The non-transitory computer readable medium of claim 35, the operationsfurther comprising: determine that the first digital audio packet hasbeen correctly received; in response to determining the first digitalaudio packet has been correctly received, sleep during the transmissionof the second digital audio packet.
 38. The non-transitory computerreadable medium of claim 35, the operations further comprising: inresponse to determining the first digital audio packet was not received,switch the receiver unit to receive the second digital audio packet onthe second frequency and wait for the second digital packet.
 39. Thenon-transitory computer readable medium of claim 35, wherein thereceiver unit is electronically coupled to or integrated in a hearingaid, earphone, or headphone.
 40. The method of claim 26, wherein thetransmission unit transmits each audio data packet three times inseparate slots of the time-slotted frame.